I. Field of the Invention
The present invention relates to a phototreating method utilizing a photochemical reaction, and an apparatus therefor.
II. Description of the Prior Art
Micropatterning of integrated circuits has been developing recently. For example, a VLSI having a minimum pattern dimension of 1 to 2 [.mu.m] was experimentally developed. Such micropatterning requires plasma etching techniques. In accordance with one plasma etching technique, a reactive gas such as CF.sub.4 is introduced into a container storing parallel-plate electrodes. A sample is disposed on one electrode (cathode) to which RF power of 13.56 [MHz] is applied, while the other electrode (anode) is grounded to cause glow discharge across the electrodes, and to generate a plasma. Cations in the plasma are accelerated by a voltage drop in the vicinity of the cathode and bombarded against the sample, thereby etching it. This technique is called reactive ion etching (RIE) and is a major technique in micropatterning.
However, in RIE, a sample to be etched is disposed in a plasma. As a result, the sample is subjected to various radiation damage such as metallic contamination from the inner surface of the chamber, in addition to destruction of an oxide film, upon bombardment, by charged particles such as ions or electrons, shifts in the threshold voltage of the device due to soft X-rays, and trap induction in the oxide film. This radiation damage can be a critical unwanted factor in making a VLSI device. Therefore, an etching technique which causes no radiation damage is sought.
Dry etching techniques, free from damage, have been reported. Such techniques include anisotropic etching of Si or poly-Si by atomic F beam having a kinetic energy of only a gas temperature in glow discharge (e.g., H. Akiya, proc, 3rd, Symp, on Dry processes, P. 119 (1981)), and etching techniques using laser or ultraviolet rays (e.g., T.J. Chuang; J. chem, phys, 74. 1453 (1981)); H. Okano, T. Yamazaki, M. Sekine and Y. Horiike, proc, of 4th Symp, on Dry processes, P. 6, (1982)). In these etching techniques, the possibility of anisotropic etching, free of damage, has been demonstrated.
According to studies of the present inventors (e.g., H. Okano, M. Sekine and Y. Horiike, proc. of 4th Symp, on Dry processes, P. 6 (1982)), in poly-Si etching in a Cl.sub.2 atmosphere by ultraviolet ray radiation emitted by an Hg-Xe lamp, anisotropic etching was found to be similar in effect to conventionally reported ion assisted etching (e.g., J. W. Coburn and H. F. Winters, J. Appl. phys, 50, 3189 (1979)). In such anisotropic etching, etching progresses much faster on a light-irradiated surface than on a non-iradiated surface. This effect is particularly notable in the etching of undoped poly-Si, monocrystalline Si, and p-type poly-Si doped with boron. A similar effect was found in the etching of n.sup.+ -type poly-Si which was doped with a high concentration of phosphorus, or, similarly, of Mo, W, Ta or a silicide thereof.
However, the above-mentioned technique has the following problems: Reactive gas radicals are optically dissociated in a gaseous phase and migrate below a mask, or, some scattered light from a surface to be etched is reflected by an inner surface of the chamber and reaches below the mask. Consequently, an undercutting 5 is formed under an etching mask 4, as shown in FIG. 1. Particularly, when a photoresist is used as the etching mask 4, since the resist is transparent to light, light irradiates a portion under the resist such that the undercutting 5 is easily formed. This undercutting 5 is a major obstacle in preventing micropatterning of the element, and is, therefore, a critical problem in VLSIs. Referring to FIG. 1, reference numeral 3 denotes a sample, such as poly-Si, to be etched; 2, an SiO.sub.2 film; and 1, an Si substrate.
FIG. 2 is a view for explaining a mechanism of anisotropic etching which has been clarified in recent studies on RIE. According to one mechanism, a recombination reaction (e.g., C. J. Mogab and H. J. Levinstein; J, Vac, Sci, Technol, 17, 721 (1980)) of Cl radicals as an etchant and CF.sub.4 radicals generated from C.sub.2 F.sub.6 as an additive gas, prevents lateral etching of an etching wall 6. According to another mechanism, a thin film 7 forming the etching mask and consisting of various kinds of unsaturated polymers such as the decomposed material of a resist, or a material formed by discharge, is deposited on the wall 6 so as to prevent penetration by the etchant (e.g., R. H. Bruce and G. P. Malafsky; E.C.S. meeting, Abs. No. 288, Denver, 1981 or Takashi Yamazaki, Haruo Okano and Yasuhiro Horiike, 30th Meeting of the Society of Applied Physics, preparatory articles, Spring Meeting, 1983).